7 Sedimentary Framework of the Western Gulf of Maine and the Southeastern Massachusetts Offshore Area

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oo) -13 75-7 Sedimentary Framework of the Western Gulf of Maine and the Southeastern Massachusetts Offshore Area

By R. N. OLDALE, ELAZAR UCHUPI, and K. E. PRADA

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Four sedimentary units ranging in age from Cretaceous to Holocene have been inferred

from a seismic survey J and a preglacial drainage system has been delineated

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1973 UNITED STATES DEPARTMENT OF THE INTERIOR

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

Library of Congress catalog-card No. 72-600378

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 Price: Paper Cover- $1.75, domestic postpaid; $1.50, GPO Bookstore. Stock No. 2401~0304 CONTENTS

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J\bstract ------1 Introduction ------1 J\cknowledgT.nents ------3 Methods of investigation ------3 Interpretation of seismic profiles ------8 Bathymetry ------3 Basement ------4 Inferred coastal-plain deposits ------4 Inferred moraine deposits ------5 Glaciomarine and marine deposits ------5 Glaciolacustrine deposits ------6 Total sediment thickness and basement structure ------6 Geologic history ------7 References ------8 Economic deposits ------9

ILLUSTRATIONS

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PLATE 1. Generalized surficial geologic map and interpretive geologic sections of seismic profiles, western Gulf of Maine and southeastern Massachusetts offshore area ------In pocket 2. Reconnaissance isopach maps of bottom sediments and structure contour map of basement surface of the western Gulf of Maine and southeastern Massachusetts offshore area ------In pocket

FIGURE 1. Map showing bathymetry of the continental margin off northeastern United States and Nova Scotia ____ 2

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SEDIMENTARY FRAMEWORK OF THE WESTERN GULF OF MAINE AND THE SOUTHEASTERN MASSACHUSETTS OFFSHORE AREA 1

By R. N. 0LDALE, ELAZAR U CHUPI,2 and K. E. P~A2

ABSTRACT (fig. 1) and consists of the Gulf of Maine, a broad 2 The sedimentary framework of the western Gulf of Maine 90, 700-km lowland, and Georges Bank, which sep­ and the southeastern Massachusetts offshore area has been arates the gulf from the open ocean. Northeast and interpreted from data obtained with a seismic profiler, supple­ Great South Channels provide passageways from the mented by information from geology on land and from sam­ Gulf of Maine to the Atlantic Ocean. Cape Cod Bay, ples offshore. A geologic map, three generalized isopach maps, Nantucket Sound, and Buzzards Bay make up and a generalized contour map of the basement surface provide significant information relative to the framework. smaller lowlands. Four sedimentary units have been distinguished on the basis The floor of the Gulf of Maine is extremely irreg­ of seismic data and are inferred to be: (1) coastal-plain ular -and is marked by deep basins, low swells, sediments of Late Cretaceous to early Pleistocene age, (2) ridges, and flat-topped banks and ledges (Uchupi, moraine deposits of Pleistocene age, (3) glaciomarine and 1968). In an early study, Johnson (1925, p. 264- marine deposits of Pleistocene and Holocene age, and (4) glaciolacustrine deposits ·of late Pleistocene age. The distribu­ 296) suggested that the Gulf of Maine was carved tion of the inferred coastal-plain deposits delineates drainage out of continental-shelf strata by stream erosion. He systems whose streams are believed to have carved the fol­ further stated that the basins within the gulf mark lowing features out of the coastal-plain and the shelf strata: positions of former stream valleys and that North­ the Gulf of Maine, the- cuesta beneath Georges Bank, and east and Great South Channels were the water gaps the islands off southern New England. This erosion is be­ lieved to have taken place during late Tertiary or early of the two major streams of the drainage systems. Pleistocene time. The inferred moraine deposits overlie base­ The topographic highs within the gulf were inter­ ment rocks or the unconformity on the coastal-plain deposits; preted by him as subordinate cuestas within the low­ they define stages in the retreat of the last glacier in coastal land. He also believed that Georges Bank was a New England. The acoustic unit assigned to the glaciomarine cuesta cut in coastal-plain strata. and marine deposits. is believed to represent in part glacial rock flour carried into the marine environment by melt-water The wide occurrence of glacial deposits in the streams and in part silt and clay winnowed from the banks Gulf of Maine and the observation that the type of and ledges during the postglacial rise in sea level. The in­ topography found off New England appears to be ferred glaciolacustrine deposits are thought to have been restricted ~ formerly glaciated coasts led Shepard, deposited in Cape Cod Bay during the retreat of the last ice Trefethen, and Cohee (1934) to suggest that glacial in that area. erosion played a significant role in the formation of the gulf. On the basis of data obtained with a con­ INTRODUCTION tinuous seismic profiler, Uchupi (1966) and Oldale Continental shelves of northern latitudes are topo­ and Uchupi (1970) suggested that the Gulf of Maine graphically very irregular and are characterized by probably was formed by a combination of preglacial lowlands along the coast, U-shaped troughs extend­ fluvial erosion and Pleistocene glacial erosion. Using ing from shore to the shelf's edge, and a chain of data from these profiles, Oldale and Uchupi (1970) banks along the outer edge of the shelf ( Holtedahl, were able to trace the preglacial fluvial systems that 1958). This type of shelf occurs off New England eroded the Gulf of Maine. Interpretation of seismic profiles also suggests that some of the basins are 1 Contribution 2669 of the Woods Hole Oceanographic Institution, based on work done under a program eondueted jointly by the U.S. Geological underlain by steeply dipping strata which acoustic­ Survey and the Woods Hole Oceanographic Institution and financed by ally resemble known Triassic sedimentary rocks in the U.S. Geological Survey. • Woods Hole Oceanographic Institution, Woods Bole, :Mass. the Gulf of Maine and the Bay of Fundy ( Tagg and

1 2 SEDIMENTARY FRAMEWORK OFFSHORE AREA, MAINE AND MASSACHUSETTS

72"

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0 25 50 75 100 NAUTICAL MILES I I I I I I I I I 0 25 50 75 100 KILOMETERS Ill II I I I I CONTOURS IN METERS

FIGURE !.-Bathymetry of the continental margin off northeastern United States and Nova Scotia. Bathymetry from Uchupi ,1965).

Uchupi, 1966; Uchupi, 1966). This interpretation continental shelf. The first area chosen for more led Oldale and Uchupi ( 1970) to speculate that the detailed study was the westernmost part of the Gulf larger basins within the gulf represent Triassic of Maine (fig. 1). The large amount of seismic fault basins modified to their present shape by profiler data collected witl:tin this relatively small fluvial and glacial erosion. area has made possible the refinement of the recon­ Most of these earlier studies were based on widely struction of the sedimentary framework of the cen­ spaced seismic profiler records ; consequently, the tral New England offshore area. Because the geo­ sedimentary framework of the offshore area could logic formations that are segregated on the basis of be reconstructed only in the broadest sense. With their acoustical-reflection properties are mostly in­ completion of a reconnaissance survey of the At­ accessible to direct observation, the geologic inter­ lantic continental margin of the United States, the pretation of seismic profiles relies heavily on in­ U.S. Geological Survey, in cooperation with the ference. Most of the unit identifications in this Woods Hole Oceanographic Institution, began a report, therefore, are referred to as "inferred." more detailed geologic mapping program of the Seismic surveys are, however, the most expedient INTRODUCTION 3 means of reconnoitering a marine area for struc­ output impedance was suitable for driving as much tural and stratigraphic information. They provide a as 600 m of tow cable, while the 20 db provided basis for planning and carrying out more specific signals greater in amplitude than any tow noise geologic investigations whose results may prove or generated by the tow cable. A broad-band monolithic disprove the inferences that were made from the amplifier with gain variable to 40 db amplified the seismic data. received signal. This signal was then fed to suitable line drivers, filtered passively, and recorded on a ACKNOWLEDGMENTS graphic recorder. During the R/V Gosnold cruise, The writers wish to thank the officers and crew of the signals were filtered in the band 150 to 250 Hz the R/V Gosnold and R/V Lulu for their coopera­ and recorded on a precision graphic recorder. During tion during the survey. Ship time aboard R/V Lulu the R/V Lulu cruise, filter settings were generally was provided by the U.S. Office of Naval Research 100 to 300 Hz, and a graphic recorder designed by (contract NONR 34-84). E. P. Curley of E.P.C. E. P. Curley was used. Recorders were set at a 0.5 Laboratories, Inc., provided a recorder of his design second scan. The air gun was triggered every fourth used during the R/V Lulu cruise. C. L. Winget, G. A. scan, and the first of the four scans was recorded. Meier, and B. B. Walden of the Woods Hole Oceano­ Navigation was by Loran A supplemented by radar graphic Institution kept the seismic system in and visual fixes. During the R/V GoS1UJld cruise, operation during the R/V Lulu cruise. Thanks are fixes were taken every 60 minutes and every 20 also extended to W. 0. Rainnie, head of the Deep minutes during the R/V Lulu cruise. Ship's speed Submergence and Engineering Section of the Ocean during both cruises was 11 km per hour. Engineering Department at Woods Hole Oceano­ graphic Institution for his help in preparing R/V INTERPRETATION OF SEISMIC PROFILES Lulu for the cruise. Zvi Ben-Avraham of Woods Hole Oceanographic Institution-Massachusetts Insti­ Seismic profiles (pl. 1) were interpreted visually tute of Technology, M. F. Kane of the U.S. Geologi­ by placing an acetate sheet over the records and· cal Survey, D. C. Jipa of the Geological Institute, tracing the individual reflectors with a grease pen­ Bucarest, Rumania, and R. D. Ballard, formerly of cil. These interpretations were then reduced visually the U.S. Office of Naval Research in Boston and now to line drawings and at the same time corrected for at Woods Hole Oceanographic Institution, partici­ variations in ship speed. Inferences as to the geologic pated in shipboard surveys. For their suggestions nature of the acoustic units were made generally on during the preparation of this report we express our the basis of known geologic history on land, from · appreciation to J. D. Milliman, K. 0. Emery, C. A. acoustic signatures of known geologic units, and, Kaye, Parke Snavely, and R. H. Meade. where possible, from rock samples offshore. General­ ized isopach maps of the sedimentary units inferred METHODS OF INVESTIGATION from seismic data, a total sediment thickness map, and a structure contour map of the basement surface The main tool for defining the sedimentary frame­ were compiled by assuming velocities of 1.5 km per work of ' the western Gulf of Maine, Nantucket sec for water, 1.5 km per sec for the inferred upper Sound, Vineyard Sound, and Buzzards Bay was a Pleistocene lacustrine and marine sediments and seismic profiler. The acoustic energy source was an Holocene marine sediments, and 1. 7 km per sec for air gun. A 5-cubic-inch chamber was used during the inferred glacial moraine and coastal-plain sedi­ the R/V Gosnold cruise, and a 10-cubic-inch chamber ments. (See pl. 2.) Although velocity data for these during the R/V Lulu cruise. During both cruises, units were not obtained during the present investi­ the gun was towed at a depth of 3m (meters) and gations, compressive velocities measured in the Gulf fired every 2 seconds. An operating pressure of of Maine (Drake and others, 1954) and along the 1,800 psi (pounds per square inch) was derived from coast (Oldale, 1969) suggest that the assumed a deck-mounted high-volume high-pressure diesel­ velocities are reasonable. driven compressor system. Signals from the bottom and subbottom reflectors were received by a 30-m, BATHYMETRY 200-element streamer array towed approximately The area of investigation is along the western 60 m astern of the vessel at a depth of 4 m. A mono­ margin of the Gulf of Maine and southeastern lithic preamplifier with a 20-decibels (db) gain was Massachusetts, enclosing within its boundaries that inserted at the head of the array. The preamplifier section of the sea floor from long 69° 40.5' to '71 °00' 4 SEDIMENTARY FRAMEWORK OFFSHORE AREA, MAINE AND MASSACHUSETTS in the southern part and from long 70°00' to 71 °00' BASEMENT in the northern part, and from lat 41 °10' to The strong reflector at the base of the seismic 43°50.2'. The most prominent topographic sub­ profiler sequence has been traced in many places to marine features in this area are Jeffreys Ledge and within short distances of the shore, where crystal­ Stellwagen Bank (pl. 1) . Both topographic highs line and sedimentary rocks of pre-Cretaceous age diverge from Cape Ann, and Stellwagen Bank ex­ (basement) crop out. With but few exceptions, this tends almost to the northern tip of Cape Cod. Depths reflector can be traced continuously throughout the along their flat tops range from less than 40 m on area, and everywhere it is inferred to represent the Stellwagen Bank to less than 60 m along most of upper surface of the basement. Jeffreys Ledge. Between the topographic highs on Subbasement acoustic reflectors were observed the seaward side is a wide delta-shaped platform east of Tillies Basin and Jeffreys Ledge (pl. 1, pro­ that includes Tillies Basin and Tillies Bank. Tillies files 309 and 321) . Their similarity to subbottom Basin has a complicated outline consisting of two reflectors in areas of known Triassic sedimentary narrow north-trending depressions on the east and rocks in the eastern Gulf of Maine (Tagg and a northwest-trending one on the west. The eastern Uchupi, 1966; Uchupi, 1966) suggests that the base­ depressions are separated by Tillies Bank. The slope ment in these areas is the same. This similarity along long 70°W. that forms the eastern side of this appears to substantiate Oldale's and Uchupi's (1970) delta-shaped platform is deeply entrenched by suggestion that the major basins in the Gulf of gullies (pl. 1) . Maine are structural features formed during the The sea floor between Jeffreys Ledge and the Triassic. Another area where subbasement reflectors coast is quite irregular and is marked by numerous were observed is the east entrance to Nantucket highs and narrow depressions. South of lat 43 o, the Sound. These reflectors may alao represent areas sea floor near the coast is much smoother, and most underlain by Triassic sedimentary rocks, but the topographic irregularities are restricted to the area proximity to Carboniferous sedimentary rocks in inside the 20-m contour. The smoothest area north the Narragansett and Boston Basins suggests that of Cape Cod occurs within Cape Cod Bay; only the these rocks may be Carboniferous. An attempt to 10-m~high Fishing Ledge disrupts the continuity of trace the Carboniferous sedimentary rocks in the the sea floor. Topographic irregularities within Nan­ Boston Basin failed, however, because subbasement tucket and Vineyard Sounds are due to shoals which reflectors were absent, and the rocks could not be are believed to be in part modified relict morainal distinguished acoustically from the igneous and ridges and sand ridges formed by longshore drift metamorphic rocks in the area. The seismic records during the last rise in sea level (Smith, 1969). On show that the surface of the basement is generally the eastern side of Buzzards Bay and in the narrow more irregular where it forms the sea bottom or tidal inlets in the Elizabeth Islands, much of the where it is overlain solely by glacial or postglacial sea floor is irregular, but because the topographic deposits than where it is overlain by sediments in­ relief of these irregularities is less than 20 m, they ferred to be coastal-plain deposits. Thus, in general, are not shown on the contour configuration of the the basement surface is more irregular north of base of plates 1 and 2A-D. In other places, the bay's Cape Ann, in Buzzards Bay, Cape Cod Bay, and parts floor is somewhat smoother. With the exception of Vineyard Sound, where it was glaciated, than of Nantucket and Vineyard Sounds, the irregularity beneath Jeffreys Ledge, Stellwagen Bank, and Nan­ of the sea floor appears to be controlled in large part tucket Sound, where it was protected from glacial by the amount of sediment cover over the basement. erosion· by coastal-plain deposits. The degree of In the area along the western side of Buzzards Bay roughness of the basement surface may also be due and west of Jeffreys Ledge, sediments are very thin to differences in erodibility of the rock units that or absent, and the sea-floor topography reflects the form the basement. morphology of the basement. A somewhat thicker cover produced the more subdued topography on the INFERRED COASTAL-PLAIN DEPOSITS sea floor west of Stellwagen Bank and in the area The basement is overlain in places by an acoustic along the eastern side of Jeffreys Ledge and Stell­ unit characterized by well-defined flat continuous wagen Bank. The relatively smooth topography of internal reflectors and. a.n irregularly undulating Cape Cod Bay, Jeffreys Ledge, and Stellwagen uppermost acoustic surface that in places truncates Bank is due to the great amount of sediment present the internal reflectors (pl. 1). The uppermost sur­ in these areas. face is inferred to be an unconformity underlain by INTERPRETATION OF SEISMIC PROFILES 5 coastal-plain sediments of Late Cretaceous to early affected by subsequent glaciation of the region. Pleistocene age. The acoustic characteristics of this These channels are probably seaward extensions of unit are similar to those on seismic records from the buried ones on land that were described by coastal-plain deposits of Tertiary age (Schlee and Upson and Spencer (1964). The valleys represent Cheetham, 1967) which underlie Fippennies Ledge streams that probably carved the lowlands forming (Uchupi, 1966) and to those on seismic records from the Gulf of Maine, Nantucket and Vineyard Sounds, erosional remnants of Cretaceous deposits on the Buzzards Bay, the cuesta beneath Georges Bank, Scotian Shelf (King and others, 1970). Additional and the islands south of Massachusetts. reasons for inferring that the acoustic unit repre­ sents coastal-plain deposits include: the fossiliferous INFERRED MORAINE DEPOSITS boulders of Eocene age (Crosby, 1879), the silicified A thick acoustically defined unit occurs in many and carbonaceous wood of Tertiary age, and glauco­ places above the erosion surface of late Tertiary or nite in the drift of Cape Cod (Oldale, 1968) ; the early Pleistocene age beneath and seaward of erosional remnants of Miocene and Eocene sediments Jeffreys Ledge and Stellwagen Bank and in the in Marshfield and Scituate and on Nonomesset areas east and south of Cape Cod. It locally overlies Island (Bowman, 1906; Emerson, 1917; Woodworth the basement surface (pl. 1) nearer shore. The upper and Wigglesworth, 1934); and the coastal-plain de­ surface of this unit is characterized by a nonsyste­ posits of Miocene and Late Cretaceous age beneath matic pattern of topographic highs and lows. Re­ Martha's Vineyard and Georges Bank (Woodworth flecting horizons within the unit are generally dis­ and Wigglesworth, 1934; Stetson, 1949; Gibson and continuous and irregular in profile, and individual others, 1968). and grouped parabolic-shaped reflectors are com­ The acoustic unit inferred to be coastal plain may mon. The unit is generally thickest in channels cut in part represent older glacial drift. If so, it is in the deposits assigned to the coastal plain. This separated from the drift of the last ice sheet by an acoustic unit is inferred to be equivalent to the thick unconformity that represents an interval of erosion accumulations of glacial drift exposed above sea during an interglacial stage or substage. This inter­ level on Cape Cod and the islands south of Cape pretation seems unlikely, however, as interglacial Cod. Although the term "moraine" is most often times are generally characterized by deposition on associated with deposits composed mostly of till the coastal plain and shelf during high sea-level (Flint, 1947, p. 126), it is used here to identify de­ stands. The deposits of Aftonian age on Martha's posits generally thought to have formed close to Vineyard (Kaye, 1964) and the presence of fossil glacial ice, including fluvial ice-contact and outwash shells in the drift of Farmdalian Age on outer Cape deposits, proglacial deltaic deposits, and basal and Cod (Oldale, 1968) indicate high stands of sea level superglacial till. The discontinuous and irregular during those interglacial times. reflectors, irregular upper surface, and parabolic Seismic data indicate that the coastal-plain de­ reflectors, believed to be boulders or gravel, are posits (pl. 2A) form the core of the major bathy­ thought to represent sediments deposited in the metric highs north of Cape Cod and underlie Nan­ chaotic depositional environments associated with tucket Sound and the islands to the south. The nearby glacial ice. The unit probably represents isopach map (pl. 2A) delineates a drainage system deposits of more than one glaciation in a manner that can be traced to the western edge of Murray similar to the Pleistocene section on Martha's Vine­ Basin, the eastern and southern margins of Nan­ yard, where deposits of all the major glacial stages tucket Sound, the mouth of Vineyard Sound, and are inferred to be present (Kaye, 1964). the entrance of Buzzards Bay. The system drain­ GLACIOMARINE AND MARINE DEPOSITS ing from the vicinity of Cape Ann may represent seaward extensions of the rivers north of this cape. Tqe uppermost seismic unit in the basins land­ The narrow depressions forming Tillies Basin ap­ ward of Jeffreys Ledge and Stellwagen Bank, in pear to be remnants of this drainage system, as do topographic lows in the basement along the coast, the gullies east of Stellwagen Bank. Other channels and locally in lows in the inferred moraine and appear to have been formed from the drainage sys­ coastal-plain deposits offshore (pl. 1) is character­ tem out of Massachusetts Bay and Cape Cod Bay. ized by a relative acoustic transparency. As this Several of these channels extend across Stellwagen characteristic best defines the unit, it is here called Bank to connect with the gullies east of this bank. the transparent layer. The unit can be penetrated The drainage system appears to have been little by high-frequency (3.5- to 12.0-kHz) as well as 6 SEDIMENTARY FRAMEWORK OFFSHORE AREA, MAI~E AND MASSACHUSETTS low-frequency (100- to 300-Hz) subbottom profiling. GLACIOLACUSTRINE DEPOSITS Numerous internal reflectors can be seen in the The upper acoustic unit in Cape Cod Bay and transparent layer, and locally, reflectors defining cut­ slightly north of the bay rests on basement in most and-fill and slump structures are present.- In some places and locally on the acoustic units inferred to places, continuous internal reflectors are absent, and be remnants of coastal-plain deposits or moraine the unit has a mottled appearance in the records. deposits. A survey of Cape Cod Bay using a high­ Generally, the unit has a flat upper surface. resolution seismic profiler (Hoskins and Knott, The thickness of the unit varies greatly because 1961) showed the unit to be composed of two parts. it is controlled in large part by the surface con­ The lower part has numerous horizontal internal figuration of the material below it. Maximum thick­ reflectors and beco~es thicker to the north. The ness found during the survey was 100 m in Tillies upper part has fewer continuous internal reflectors Basin. In many other places, thickness exceeds and locally is characterized by internal parabolic 30-40 m. reflectors. This upper part thickens toward the Sediments in the uppermost part of this trans­ south. Hoskins and Knott (1961) inferred that the parent layer have been extensively sampled and lower part represented coastal-plain deposits of are known to consist of silt and clay of Holocene Tertiary age. They believed that the upper part was age. In Boston Harbor, boreholes near 3.5-kHz sub­ glacial drift of Pleistocene age and that the para­ bottom profiles showed the upper part of the layer bolic reflectors represented boulders or gravel with­ to be silt and clay of Holocene age and the lower in the deposit. As no reflecto~s that might represent part to be glaciomarine silt and clay of late Pleisto­ an unconformity were observed between the upper cene age (R. H. Burroughs, oral commun., 1970). A and lower part, we have inferred that the acoustic similar correlation is inferred for the transparent unit represents deposits wholly of Pleistocene age, layer offshore. The mottling shown in the records is perhaps glaciolacustrine sediments deposited in inferred to represent local deposits of coarser Cape Cod Bay during the retreat of the last ice. grained material, perhaps sand and gravel, within the silt and clay. The silt and clay represented by TOTAL SEDIMENT THICKNESS AND BASEMENT the transparent layer thus have a complex origin. STRUCTURE Initially, most of the material consisted of glacial Total sediment thickness and depth to basement, rock flour carried to the sea by melt-water streams as determined from seismic data, are shown on plate and deposited in the basins. Later, as the rising 2C and D. Much of the sea floor adjacent to the Holocene sea submerged the banks and ledges, silt coast as far south as Plymouth consists of closely and clay were winnowed from glacial drift by wave spaced topographically high basement outcrops sep­ action and carried into the basins by currents. arated by topographic lows containing sediment less Finally, the upper part of the silt and clay deposit than 20 m thick. Seaward of this coastal zone and probably contains some material carried to the sea south of Plymouth the sediments gradually thicken, by modern streams. and outcrops of the basement are absent. The sedi­ No internal reflector within the transparent layer ments are thickest beneath Jeffreys Ledge, Stell­ could clearly be inferred to be an unconformity be­ wagen Bank, Cape Cod, Nantucket Sound, and the tween the glaciomarine and Holocene marine de­ islands farther south. Within these areas the greatest posits. This fact suggests that in most places, off­ thicknesses occur in narrow troughs. A well-devel­ shore deposition has been continuous since late oped drainage system is defined by both maps partic­ Pleistocene time. ularly south of Cape Ann. Many of these valleys Contact relationships between the upper Pleisto­ appear to be offshore extensions of major streams cene to Holocene mar~ne deposits and moraine de­ of present drainage systems, that is, extensions of posits are complex. The moraine deposits represent the Merrimack River and the streams that drain in part subaerial facies of the glaciomarine deposits. Boston Harbor. Other submarine valleys like the ones Near the contacts between these units the sediments off Plymouth and in Nantucket and Vineyard Sounds may grade from one to the other. Similarly, the may be offshore extensions of preglacial river valleys marine deposits of Holocene age, although strati­ now buried by glacial drift on land. As many of the graphically younger, grade laterally into reworked submarine valleys are wholly or in part filled with glacial drift on the topographic highs. Thus, in many sediment inferred to be moraine, the drainage is places, a reflector representing the contact between thought to be preglacial. The streams carved the these units was not present in the seismic records. lowlands and cuestas out of the continental shelf GEOLOGIC HISTORY 7 strata. North of Cape Ann, the seismic lines are far The glacial chronology of New England is not apart, and only one major drainage system could be completely understood. Most workers recognize evi­ delineated. dence for two glacial advances separated by an inter­ glacial stage or substage (Schafer and Hartshorn, GEOLOGIC HISTORY 1965). Others have suggested that all glacial stages are present in the region (Kaye, 1961, 1964). Glacial On the basis of seismic data obtained during the stages within the deposits offshore cannot be recog­ nized in the seismic data. Older drifts and inter­ present study and supplemented by information glacial deposits may make up part of the sediments available on land, the geologic history of the western inferred to .be coastal-plain deposits. More probably Gulf of Maine, Cape Cod, Nantucket and Vineyard. they may occur as part of the sediment labeled as Sounds, Buzzards aay, Nantucket, and Martha's moraine deposits, in a manner similar to that de­ Vineyard can be summarized as follows. Until scribed by Kaye (1964) for the Pleistocene section Cretaceous time the history of the area may have on Martha's Vineyard. As such distinctions within been similar to that indicated by the onland geology. the aeoustic unit inferred to be moraine are not The sedimentary, tectonic, and erosional cycles that possible, the glacial history presented here describes left their imprint on land should also have played events associated with the last glaciation of the a part in the formation of the basement now present area, which occurred in Woodfordian time. offshore. During most of Paleozoic and early Mesozoic time, New England was an area of plutonic Radiocarbon dates on material in the drift on and volcanic activity. During the Carboniferous, outer Cape Cod (Zeigler and others, 1960; Oldale, narrow fault troughs that filled with continental 1968) suggest that the last ice advanced into south­ deposits developed in some places. Boston Basin eastern New England 20,000 to 26,000 years ago. represents one such trough that extends offshore. A Fossil material, glauconite, and the paucity of more extensive series of fault troughs that filled feldspar in the drift (Oldale, 1968) also suggests with continental sediments formed during the that the ice overrode shelf sediments of Farmdalian Triassic, and many of the larger basins in the Gulf Age as well as older glacial drift and coastal-plain of Maine are thought to be offshore equivalents of sediments. The maximum advance of the ice was the Triassic basins on land. to Martha's Vineyard and Nantucket (Schafer and Hartshorn, 1965; Pratt and Schlee, 1969), where Upper Cretaceous continental deposits on Martha's large terminal moraines were built by the Buzzards · Vineyard (Woodworth and Wigglesworth, 1934) Bay, Cape Cod Bay, and possibly the South Channel suggest that much of the area may have been above lobes. sea level during Late Cretaceous time. Meanwhile, marine deposition was taking place along the shelf's Retreat of the ice from this maximum position edge as attested by the presence of Upper Cretaceous began sometime after 15,300 ± 800 B.P., as shown marine sediments in the submarine canyons south by a radiocarbon date (Kaye, 1964) on leaves found of Georges Bank (Stetson, 1949). In early Tertiary stratigraphically below till and outwash. Retreat of time, the Gulf of Maine and the shelf to the west the lobes was not entirely synchronous. The Buz­ were submerged; the transgression of the sea zards Bay lobe retreated somewhat faster than the reached its peak during the Miocene, as indicated Cape Cod Bay lobe (Schafer and Hartshorn, 1965), by the Miocene glauconitic sediments along the and the Cape Cod Bay lobe in turn retreated rather coast south of Cape Ann (Bowman, 1906; Emerson, more rapidly than the South Channel lobe. The dif­ 1917). In late Tertiary or early Pleistocene time, ferential retreat of the lobes may have been caused sea level dropped, or the region was uplifted. by differences in the depth and size of the basins, Streams extended their courses across the shelf and the srnallest of which is Buzzards Bay and by far the carved a series of lowlands beneath Buzzards Bay, largest and deepest of which are Great South Chan:­ Vineyard and Nantucket Sounds, and the Gulf of nel and the basins to the north. Maine, and they carved cuestas beneath the Eliza­ The first major stand in the retreat is marked beth Islands, Martha's Vineyard, Nantucket, Stell­ by the outwash and moraines on inner Cape Cod wagen Bank, Jeffreys Ledge, and Georges Bank. that were deposited by the Buzzards Bay and Cape Erosional remnants were left beneath the smaller Cod Bay lobes. During this time, the extremities of banks within the Gulf of Maine. Great South and the lobes were in positions approximated by the Northeast Channels were the water gaps of much west and north shore of Cape Cod. The South Chan­ of this drainage system. nel lobe occupied a more southerly position, so that 8 SEDIMENTARY FRAMEWORK OFFSHORE AREA, MAINE AND MASSACHUSETTS its western edge was east of Nantucket. The possible deposition of moraine sediments over the coastal­ formation of a lake between the ice lobes and the plain sediments northeast of Stellwagen Bank and islands to the south is indicated by the thick clay east of Jeffreys Ledge. layer in the subsurface of southeastern Cape Cod By about 12,000 years ago the western part of the (Koteff and Cotton, 1962). The lake, if it existed, Gulf of Maine was free of glacial ice. As the region was probably dammed to the east by the South was submerged, the glacial deposits atop the banks Channel lobe and to the south and west by drift. were reworked. The fines winnowed out of these Progradation of outwash and deltaic sediments in a southerly direction appears to have filled the lake. deposits came to rest in the deeper and quieter water of the basins east and west of the topographic highs. Retreat of the ice northward from Cape Cod re­ The late Pleistocene high stand of sea level along sulted in a proglaciallake in Cape Cod Bay, dammed much of coastal New England was due to the de­ to the south and west by drift and to the east by pression of the crust by ice loading. Sea level may the South Channel lobe. Initially the lake was nar­ have been as high as 37 m above the present level row, and a minor stand of the ice resulted in deltaic (Bloom, 1960; Kaye and Barghoorn, 1964). During and lacustrine deposits along the north shore of this submergence a gray silt-clay that contains a Cape Cod. Smaller moraines south of Plymouth cold-water marine fauna was deposited along the (Schafer and Hartshorn, 1965) and a part of the coast (pl. 1). In response to ice unloading, the crust drift on outer Cape Cod, deposited into the lake by rebounded and sea level dropped again. In Boston melt water from the South Channel lobe, may cor­ this emergence occurred 11,000 years ago when sea relate with this stand. Continued retreat of the ice level dropped 21 m below its present level (Kaye from Cape Cod Bay to a position north of Duxbury and Barghoorn, 1964). Farther north in Maine the allowed the lake to expand. Probably at this time, emergence occurred 4,200 years ago, and sea level melt water from the South Channel lobe built most dropped 2 m below its present level (Bloom, 1960). of the outer Cape, and to the west, melt water de­ After this emergence, sea level rose again, reaching posited sediments to form the deltas at Duxbury to nearly its present level several thousand years (Chute, 1965). Fine-grained sediments from the ago. During this last rise in sea level, deposition by Cape Cod Bay and South Channel lobes were de­ littoral drift resulted in the formation of the posited in the lake, while the Cape Cod Bay lobe northern tip of Cape Cod (Zeigler and others, 1965), contributed coarse material to the lake sediments as well as the barrier islands and spits along the from ice or by rafting. As the western side of the New England coast, while modern streams have con­ South Channel lobe retreated away from the outer tributed silt and clay to the bathymetric lows. cape, the lake drained by way of Little Stellwagen Basin. The glacial sediments of Cape Cod and in Cape ECONOMIC DEPOSITS Cod Bay were deposited sometime before 14,000 years ago, as by then the ice front had retreated to The relatively thin sediment cover indicated by a position north of Boston (Kaye and Barghoorn, the seismic profiles in the western Gulf of Maine, 1964) and the Cape Cod region was submerged. Nantucket and Vineyard Sounds, and Buzzards Bay From the time when the drift was deposited on the demonstrates that the petroleum potential within outer cape, stages in the retreat of the South Channel the coastal-plain sediments appears to be non­ lobe are not well known. However, the glacial de­ existent. The mineral resources of the region are posits in Stellwagen Bank probably represent deltas probably restricted to sand and gravel. The geologic formed in a marine environment. The finer grained map compiled from the seismic profiles indicates glaciomarine sediments (rock flour) were deposited that the best sites for these deposits are the top of in Stellwagen Basin, possibly from melt water of the banks and ledges. Smaller concentrations of sand South Channel lobe, as the seismic recordings from and gravel are found locally along the coast and in the bank suggest an eastern source for these de­ Cape Cod Bay. In many places these smaller concen­ posits. The glacial deposits atop Jeffreys Ledge and trations are covered by a variable thickness of the fine-grained sediments in the basins west of the marine mud of Holocene age. The large basins ap­ ledge probably have a similar origin. Continued pear to contain little gravel, as in most places the shrinkage of the South Channel lobe resulted in the transparent layer rests directly on basement. REFERENCES 9

REFERENCES Koteff, Carl, and Cotton, J. E., 1962, Preliminary results of recent deep drilling on Cape Cod, Massachusetts: Science, Bloom, A. L., 1960, Late Pleistocene changes of sea level in v. 137, no. 3523, p. 34. southwestern Maine: Augusta, Me., Maine Geol. Survey, LaForge, Laurence, 1932, Geology of the Boston area, Massa­ 143 p. chu:;etts: U.S. Geol. Survey Bull. 839, 105 p. Bowman, Isaiah, 1906, Northward extension of the Atlantic Oldale, R. N ., 1968, Geologic map of the Wellfleet quadrangle, preglacial deposits: Am. Jour. Sci., 4th ser., v. 22, p. Barnstable County, Cape Cod, Massachusetts: U.S. Geol. 313-325. Survey Geol. Quad. Map GQ-750. Bradley, Edward, 1964, Geology and ground-water resources --- 1969, Seismic investigations on Cape Cod, Martha's of southeastern New Hampshire: U.S. Geol. Survey Vineyard, and Nantucket, Massachusetts, and a topo. Water-Supply Paper 1695, 78 p. graphic map of the basement surface from Cape Cod Chute, N. 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C., 1949, The sediments and stratigraphy of the Alaska: Jour. Geology, v. 66, no. 4, p. 461-471. east coast continental margin-Georges Bank to Norfolk Canyon: Massachusetts Inst. Technology and Woods Hoskins, Hartley, and Knott, S. T., 1961, Geophysical in­ Hole Oceanog. Inst. Papers in Physical Oceanography vestigation of Cape Cod Bay, Massachusetts, using the and Meteorology, v. 11, no. 2, 60 p. continuous seismic profiler: Jour. Geology, v. 69, no. 3, Tagg, .A. R., and Uchupi, Elazar, 1966, Distribution and p. 330-340. geologic structure of Triassic rocks in the Bay of Fundy Johnson, D. W., 1925, The New England-Acadian shore line: and the northeastern part of the Gulf of Maine: U.S. New York, John Wiley & Sons, 608 p. Geol. Survey Prof. Paper 550-B, p. B95-B98. Kaye, C. A., 1961, Pleistocene stratigraphy of Boston, Massa­ Uchupi, Elazar, 1965, Map showing relation of land and chusetts: U.S. Geol. 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chusetts-Coastal Plain and surficial deposits: U.S. Zeigler, J. M., Hoffmeister, W. S., Giese, Graham, and Tasha, Geol. Survey Misc. Geol. Inv. Map I-514-B, scale 1:250,- Herman, 1960, Discovery of Eocene sediments in the 000, 8 sheets. subsurface of Cape Cod, Massachusetts: Science, v. 132, Woodworth, J. B., and Wigglesworth, Edward, 1934, Geo­ no. 3437, p. 1397-1398. graphy and geology of the region including Cape Code, Zeigler, J. M., Tuttle, S. D., Tasha, H. J., and Giese, G. S., the Elizabeth Islands, Nantucket, Marthas Vineyard, No 1965, The age and development of the Provincelands Mans Land and Block Island: Harvard Coli. Mus. Comp. Hook, outer Cape Cod, Massachusetts: Limnology and Zoology Mem., v. 52, 338 p. Oceanography, v. 10, supp., p. R298-R311.

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